When an optical disc drive writes information on a recordable optical disc or reads information that is stored there, the light beam needs to always have a predetermined focusing state on a target track on its storage layer. For that purpose, “focus control” and “tracking control” need to be done. The focus control means controlling the position of an objective lens perpendicularly to the storage layer such that the focal point of the light beam is always located right on the storage layer. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of the optical disc such that the focal point of the light beam is on a predetermined track.
The optical disc drive further needs “tilt control”, which means either controlling the angle of an optical pickup such that the optical axis of the light beam is perpendicular to the storage layer of the optical disc or controlling the angle of the objective lens such that the magnitude of coma aberration to produce at the focal point of the light beam becomes substantially equal to zero (e.g., within 14 mλ).
Supposing a situation where a light beam is incident on an optical disc with its optical axis forming right angles with the storage layer of the optical disc is a reference state, the difference of the actual angle formed by the optical axis with respect to the storage layer from the right angles is called a “tilt angle”. The tilt control is done to keep the magnitude of a coma aberration, which is produced at the focal point of the light beam as being affected by this tilt angle, substantially equal to zero (e.g., within 14 mλ). A state in which the magnitude of coma aberration is not equal to zero will be referred to herein as a “tilted state”.
As used herein, a “disc tilt” is defined to be a tilt caused by an upward or downward deformation of a disc and a “lens tilt” is defined to be a tilt caused by an inclination of an objective lens.
The control of changing the angles of the objective lens described above is a method of reducing the total tilt quantity to zero by offsetting a disc tilt by a lens tilt. According to another method, the tilt may also be controlled by varying the phase of the light beam using a liquid crystal display device, for example.
As the storage density of a storage layer in an optical disc increases, it has become more and more important to perform a tilt control precisely enough to avoid causing tilts. This is because once a tilt has been caused, the quality of a written signal or the jitter of a read signal will deteriorate too significantly to keep the performance of the optical disc drive good enough.
FIGS. 12(a) and 12(b) schematically show cross sections of a light beam on the storage layer of an optical disc in a situation where no tilt has been caused and in a situation where a tilt has been caused, respectively. Meanwhile, FIGS. 13(a) and 13(b) show how the jitter of a read signal changes with the tilt quantity (which is typically represented by the tilt angle) and how the error rate changes with the tilt quantity. As shown in FIGS. 12 and 13, the tilt produces a coma aberration, thus deteriorating the jitter and increasing the error rate in the read signal.
If the aberration produced by the tilt exceeded its tolerance (e.g., approximately 0.3 degrees for DVDs), information could no longer be read or written in the best condition, thus decreasing the reliability of the information, which would be a problem. Also, the tolerance range of the aberration with respect to the tilt angle is inversely proportional to the third power of the numerical aperture (NA). That is why the higher the storage density of an optical disc, the narrower the tolerance range and the more difficult it would be to keep the read/write performance of the drive good enough. This is because to increase the storage density, the beam spot size of the light beam should be decreased. For that purpose, the light beam should have its wavelength shortened and its numerical aperture (NA) increased.
To realize good read/write performance on high-density optical discs, the optical disc drive disclosed in Patent Document No. 1 adopts not only mechanical positioning control on an optical system and a drive system but also a tilt control for appropriately tilting either an optical head or an objective lens. During the tilt control, the optical disc drive measures the magnitudes of focus drive at two different radial locations to detect the tilt angle caused by the upward or downward deformation of the disc or a misalignment of the mechanical chassis. And the optical disc drive corrects the tilt angle detected and reduces it to as close to zero as possible, thereby realizing read and write operations in an appropriate condition.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2003-281761        